1. Field of the Invention
The teachings herein relate to a hand held detector of ionizing radiation and more particularly to a detector for discriminating a gamma component and a neutron component.
2. Description of the Related Art
Detection of radioactive materials, particularly those illicitly hidden in the stream of commerce, requires a variety of radiation detection equipment be available. In particular, Hand-Held RadioIsotope Identification Devices (HHRIID) are needed in the field to quickly determine the presence of special nuclear material (in simple terms, being defined as plutonium, uranium 233, uranium enriched in the isotope 233 or in the isotope 235). Preferably, HHRIID provide users with information regarding the radioisotopic composition of any radioactive material identified. Typical HHRIID devices are handheld and battery-powered and have limited features. These devices are consistently exposed to variable operating conditions and radiation backgrounds. Further, for most applications, the HHRIID must satisfy the specifications set forth in ANSI Standards N42.33 and N42.34. Since special nuclear materials can present a mixed neutron-gamma signature, effective isotopic analysis requires an HHRIID be capable of providing a high-resolution, high-sensitivity response to gamma rays without being affected by the presence and intensity of a neutron field. This poses a problem in HHRIID embodiments in which, for practical reasons of reducing size, weight and power consumption, a single photosensor detects the light emitted by both the gamma ray sensing element and the neutron sensing element. A method to discriminate between the scintillation light pulses emitted by the two radiation sensing elements is necessary.
For detectors without pulse shape discrimination, the energy spectrum of one type of radiation can be distorted by the presence of a strong field of radiation of a second type. For example, the gamma ray spectrum, which is very important for radionuclide identification in HHRIID applications, can include counts due to unrejected neutron response and thus reduce the sensitivity and specificity of the device.
Some techniques for radiation pulse shape discrimination addressed the problem by measuring and comparing features of the electrical signals emitted by the photosensor after light excitation: pulse amplitude, pulse rise time, pulse decay time, or total integrated charge. Some applications of the existing methods include, for example: 1) discrimination against gamma background in liquid or solid organic scintillators used as direct fast neutron detectors; 2) discrimination between long-range and short-range particles in gas proportional counters; 3) separation of radiations of different energies and/or depths of interaction in phoswich detectors (that is, a detector that includes a combination of scintillators with dissimilar pulse shape characteristics optically coupled to each other and one or more photomultiplier tubes); 4) rejection of spurious or defective pulses in direct conversion detectors (Si, HPGe), and other.
One attempt to address some of the challenges in detection is disclosed in U.S. Pat. No. 6,953,937, entitled “Method and Apparatus for the Detection of Neutrons and Gamma Rays,” which issued on Oct. 11, 2005 to Reber et al. This patent teaches a pulse discrimination method for discriminating between pulses having a short decay period and a long decay period, may comprise: Detecting the pulse; integrating a rise portion of the pulse; integrating a decay portion of the pulse; and comparing the integrated rise portion of the pulse with the integrated decay portion of the pulse to distinguish between a pulse having a long decay period and a pulse having a short decay period. Unfortunately, the teachings of this patent do not provide for sensitive analyses, as a single detector is used, and calls for an “artificial line of separation” to distinguish radiation types. Further disadvantages of this design include poor sensitivity and energy resolution for gamma radiation as well as optical self-absorption and self-excitation in the scintillator material.
What is needed is a compact, integrated HHRIID design that provides for accurate discrimination between radiation types, thus enabling improved analyses of the various components of a mixed radiation field.